1. Field of the Invention
The present invention relates to an organic electroluminescent element (hereinafter called an "organic EL") having a fixed display pattern realized by a plurality of fixed display segments each of which is composed of a laminate formed by sequentially laminating an anode, a hole transporting layer made of an organic compound, a luminescent layer made of an organic compound and a cathode.
2. Description of the Related Art
A portion of known organic EL has a so-called double-layer structure formed as shown in FIG. 3A such that two layers consisting of a luminescent layer 1 made of a thin organic fluorescent film and an organic hole transporting layer 2 are laminated between a metal electrode (for example, Mg:Ag or Al:Li) 3 serving a cathode and a transparent electrode 4 serving an anode. Another structure called as a triple-layer structure has been known in which an organic electron transporting layer 6 is, as shown in FIG. 3B, laminated in addition to the organic hole transporting layer 2 and the luminescent layer 6 of the above-mentioned double-layer structure.
Although the organic EL of the above-mentioned type is able to emit light having high luminance (for example, 1000 cd/m.sup.2 or higher) even with low voltage (for example, 10 V or lower), an excessively high electric current flows such that an electric current of 1A/cm.sup.2 or higher sometimes instantaneously flows. Therefore, resistance raised in a portion of the wiring which reaches the light emitting portions for forming a display pattern enlarges a drop of the voltage. This leads to a fact that the light emitting portions cannot be applied with the same voltage and thus luminance becomes irregular.
As the material of the anode line, ITO (Indium-Titanium Oxide) has mainly been employed. The reason for this is that light permeability and a work function larger than 4.5 eV are required for the material to obtain a satisfactory hole injection efficiency. Although other materials, such as Au, Pt and Ni, are considered feasible to be employed, each of these materials requiring a small thickness to realize transparency involves an excessively raised sheet resistance. Therefore, ITO has been employed in actuality.
Since the resistance in the electric circuit is raised if ITO having high specific resistance is employed, a structure disclosed in Japanese Patent Laid-Open No. 6-5369 has been employed. That is, the organic EL of the foregoing type, as shown in FIG. 4A, has a light emitting portion composed of a transparent anode 23 and a metal cathode 24 forming a pair which holds a hole transporting layer 21 and a luminescent layer 22. Moreover, a light emitting portion formed at an intersection region in which the transparent electrode 23 and the metal cathode 24 are disposed opposite to each other, as one unit, forms one pixel so as to perform dot matrix display. A double-layer wiring structure is formed such that a metal film 25 having a smaller working function than that of the transparent anode 23 is laminated between the transparent anode 23 and the hole transporting layer 21 in order to weaken the overall resistance.
However, the organic EL disclosed in Japanese Patent Laid-Open No. 6-5369 has no contrivance for preventing change in the wiring resistance in each light emitting portion. A consideration can be performed that the irregular luminance distribution is prevented by employing a structure formed such that a constant current circuit 26 formed as shown in FIG. 4B is connected to each of the lines connected to the light emitting portions.
As described above, the structure of the organic EL disclosed in Japanese Patent Laid-Open No. 6-5369 requires the constant current circuit 26 to be connected to each line in order to prevent irregular luminance distribution among the light emitting portions occurring attributable to change in the wiring resistance. Thus, there arises a problem in that a large cost is required.
If the light emitting portion is formed into a fixed display segment as is employed in the fixed pattern display structure and thus the fixed display segments have individual display areas, the electric lines from the voltage supply terminals to the fixed display segments have different lengths. Therefore, since different electric currents flow in the fixed display segments even if the voltage supply terminals have the same potential, the electric lines encounter different amounts of voltage drops. Therefore, the light emitting portions encounter irregular luminance distribution.
To overcome the above-mentioned problem, the electric currents to be allowed to flow in the electric lines of the fixed display segments must be controlled. In this case, there arises a problem in that the structure of the circuit becomes too complicated. If a resistance correction element is provided for the inside portion of the drive circuit, the cost is raised in this case.
If the resistance is not corrected, a voltage drop of 1 V takes place in a case where voltage of 10 V is applied, an electric current level (the luminance) of 1 mA is required and the resistance raised from the electric line made of ITO is 1 kW. If the voltage drop of 1 V takes place, a difference in the luminance to an extent of hundreds of cd/m.sup.2 takes place.